Quick design explanation
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Here is a quick explanation of the design where each different part is described in a few words. For more information, read the paper Realisation of a low noise acquisition board : design choices. All the design has been done on KiCAD. For a better experience, it is advised to download the last KiCAD project from the repository and explore it.
The whole board
On this card will be connected all the future analogue time measurement elements. The output of these elements has first to be converted in numerical data before being processed: that is where the ADC board comes in. As we want the best possible resolution for the ADC, we pay extremely attention to the noise generated by other components. This board is a carrier card for the MicroZed board which will be used to process incoming data.
The design schematics are divided into 5 different parts :
> # the ADC and the inputs,
> # the clock circuitry,
> # the MicroZed Board connections,
> # the power supply
> # the miscellaneous
This first picture, the top level of the schematics, is only to show the different connections between these different parts. In addition to the power lines and data lines, we can see the SPI bus between the MicroZed and both the ADC. This bus can be used to configure the ADC.
The ADC and the Inputs
The AD9253 is able to sample data at a rate of 125 MHz. Its resolution is 14 bits.
The AD9253 has four inputs. On the previous verion of the board, we chose the AD9645 which has more or less the same properties. This one has only two channels. Using two ADC, even if they are synchronized with the same sampling clock, may increase the error jitter on the measurements. It is in order to reduce this jitter that we finally chose to use the AD9253.
A particular attention is given to the power supply decoupling. Multiple capacitors with multiple values are placed to reduce the impedance on a large bandwidth and then avoid the noise to disturb the measurements. This has been done as well for the analog power lines as for the digital ones. To have the lowest ESR (Equivalent Serie Resistor) and the highest SRF (Self Resonant Frequency), we only use ceramic capacitors (C0G/NP0 when possible, otherwise we use X7R).
All the resistors are in metal film to give the best performance (less noisy, lower temperature coefficient).
On each input are one SMA connector for ground referenced signals and two others for differential ones. All inputs have an impedance around 50 ohm and a bandwidth of 400 kHz to 175 MHz.
The Clock Circuitry
By default, the sampling frequency is 100 MHz given by the local
oscillator. Another frequency can be set using an external oscillator.
By this way, we can set the frequency in a range from 5 MHz up to 125
MHz.
The clock splitter (LTC6957) is used to send the clock signal both to
the ADC and to a trigger (see miscellaneous part). . This component can
also filter the input clock signal. The parameters of the filter can be
set using the FILTA
and FILTB
inputs. To be able to change the
filter and to adjust it to our needs, we put these inputs on
jumpers.
The power Supply
We need enough power to supply all the components on the board but also the MicroZed board. Furthermore, these components need different voltage levels.
The first stage is a buck converter. It is used to convert the voltage from 12 V to 5V DC. These 5 V are needed by the MicroZed board.
We put two different connectors for the power : a DC barrel jack and a MINI DIN 4. To be sure to have the right voltage polarity, we put a double rectifier bridge using Schottky diodes. These diodes are chosen to avoid a too high voltage drop at the input. A diode is also placed next to the barrel jack to protect the circuit in case the polarity is inverted.
The LM43603 has been chosen because its switching frequency can be set up to 2.2 MHz. In addition, this frequency can be changed using an external clock. These high frequencies are easier to shield than lower ones. A further reason for choosing this buck converter is that its switching MOSFETS are inside the chip package, reducing considerably the "hot loop".
Instead of using big capacitors at the input and the output of the converter, we decided to use different values to catch the high harmonics and then improve the noise rejection.
We then convert these 5 V in a 3.3 V one. This last is used to supply the clock splitter. Finally, two LDO are used to regulate the digital and analog power supplies of the ADC.
External connections
The connections to the MicroZed board are done via its FCI connectors. All these inputs can be configured to work in LVDS. As the outputs of our ADC are already in LVDS, this configuration fits ideally with our project.
There are two connectors. Only one is represented here. The other connector is powered with the 3.3V and is used to communicate with the components in the miscellaneous part and the external GPIO ports.
In addition to these connections to the ADC, we also provide the board other connections to be able to use the other MicroZed GPIO.
Miscellaneous
Some other components have been added to the board. First, we have the trigger. This trigger is used to send a clock synchronized signal to an external device. We also have a thermometer to be able to measure the card temperature. This chip has also a unique ID and allow to make the difference between similar boards.
Nicolas Boucquey 19th of October 2016